Decanting type centrifuges employ a bowl which rotates about a horizontal or vertical axis and contains a helical scroll conveyor to separate a slurry fed thereto into its constituent solids and liquid. The helical conveyor rotates at a slightly different speed within the bowl to scroll the heavier solids to discharge ports at a smaller diameter end of the bowl. The separated liquid flows in the opposite direction and is discharged from ports at the opposite end of the bowl. The decanter can be of two principal types, either solid bowl or screen bowl. In the latter, the solids are scrolled by the conveyor over an additional perforated screen section attached to the smaller diameter end of the bowl prior to discharge.
Existing decanter centrifuges of both the solid screen bowl types operate when fed with a slurry containing solids with a higher specific gravity than the liquid constituent of the slurry, either to:                (a) separate the solid particles from the liquid, or        (b) classify the solids, that is to divide the solids so that particles above a certain size are discharged as solids and particles below that size are discharged with the liquid.        
For both separation and classification, the rotation of the decanter applies centrifugal force to the slurry to promote rapid settling of the higher specific gravity solids for scrolling and discharge. Hereinafter, the words ‘separate’ and “separation” when applied to solids and liquids, include ‘classify’ and ‘classification’.
FIG. 1 of the accompanying drawings shows, in part section, a conventional solid bowl decanter operating assembly designed to rotate about a horizontal axis XX and to separate slurry fed via a feed pipe 1 and feed ports 2 into a bowl 3, which includes a cylindrical section 3A joined to a section 3B shaped as a frustrum of a cone—herein referred to as the conical bowl section. The slurry, subjected to centrifugal force fills the bowl up to an inner surface level 4 determined by the radial position of liquid outlet ports 5. A conveyor hub 6, coaxially mounted within the bowl 3 and supported on bearings 7, carries scrolling flights 8 wound in a helix and attached to the hub 6. The plane of the scrolling flights tilts forward to subtend an angle (a), typically 1°-7°, from a perpendicular line from the wall of the cylindrical 3A or the conical 3B sections of the bowl 3. A gearbox (not shown) drives the conveyor 6 in the same direction of rotation but at a speed slightly different from that of the bowl 3 such that, relative to the bowl, the flights 8 scroll the solids towards the solids end 9 of the decanter and discharge the solids 10 through solids outlets 11. The gearbox and rotating assembly are mounted in bearings (not shown) and rotated by a drive motor. Under centrifugal force, the solids 10 settle rapidly on the bowl inner wall and are scrolled by the conveyor flights 8 and discharged from the solids outlet 11 whilst the liquid, after primary separation, flows from the outlet ports 5.
FIG. 2 of the accompanying drawings shows in part section and to a larger scale a perforated cylindrical screen section 12 added to and extending the smaller diameter end of the conical section 3B, an extended conveyor hub 6 and extended scrolling flights 8. These additions convert the solid bowl decanter to the conventional screen bowl type and provide further separation by filtration following the primary separation in the solid bowl sections 3A and 3B as the liquid flows through perforations/slots 16.
Some processes using decanting type centrifuges require solids to be washed after the primary separation to reduce liquid retained as a thin film on the surface of the solid particles or displace this liquid with another. In the present state-of-the-art, the capacity to achieve this in both the solid and screen bowl decanters is limited. For the solid bowl, wash liquid is supplied by a wash pipe 13 fitted within or along the slurry feed pipe 1, as shown in FIG. 1, and flows through wash ports 14 in the hub to the solids bulk 10 being scrolled along the conical section 3B. The extent to which the solids are washed is limited by the tendency of the wash liquid to flow over the sloping surface 15 of the solids rather than through the bulk of the solids 10, then to flow down the helix to mix with the separated mother liquid. Also the wash liquid flow is concentrated at the wash ports giving uneven washing.
Similarly, and for the same reason, the capacity of the screen bowl decanter to wash separated solids is limited by flow over the surface 15 of the solids, as shown in FIG. 2, and by the uneven wash liquid distribution. Furthermore, any flow through the bulk of the solids carries with it fine solid particles (defined as solids that could pass through to screen perforations/slots 16) that would otherwise remain trapped in the bulk of the solids and be discharged at the solids outlet 11 by the conveyor 8.
An object of the present invention is to seek to improve the washing effectiveness and efficiency of both solid bowl and screen bowl decanting centrifuges.